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Oracle® Database Backup and Recovery User's Guide
11g Release 1 (11.1)

B28270-03
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6 Configuring the RMAN Environment: Advanced Topics

This chapter describes how to perform setup and configuration tasks. This chapter contains the following topics:

Configuring Advanced Channel Options

While "Configuring Channels" explains the basics for configuring channels, this section explains more advanced channel topics. This section contains the following topics:

See Also:

"RMAN Channels" for a conceptual overview of configured and allocated channels, and Oracle Database Backup and Recovery Reference for CONFIGURE syntax

About Channel Control Options

Whether you allocate channels manually or use automatic channel allocation, you can use channel commands and options to control behavior. Table 6-1 summarizes the ways in which you can control channel behavior. Unless noted, all channel parameters are supported in both CONFIGURE CHANNEL and ALLOCATE CHANNEL commands.

Table 6-1 Channel Control Options

Type of Channel Control Commands

Limit I/O bandwidth consumption

You can use the RATE channel parameter to act as a throttling mechanism for backups.

Limit backup sets and pieces

You can use the MAXPIECESIZE channel parameter to set limits on the size of backup pieces. You can also use the MAXSETSIZE parameter on the BACKUP and CONFIGURE commands to set a limit for the size of backup sets.

Vendor-specific instructions

You can use the PARMS channel parameter to specify vendor-specific information for a media manager. You can also use the SEND command to send vendor-specific commands to a media manager.

Channel parallelism for backup and restore operations

You can use CONFIGURE DEVICE TYPE ... PARALLELISM for persistent channel parallelism or multiple ALLOCATE CHANNEL commands for job-level parallelism.

Connection settings for database instances

You can specify which instance performs an operation with the CONNECT channel parameter.


See Also:

Oracle Database Backup and Recovery Reference for ALLOCATE CHANNEL syntax, and Oracle Database Backup and Recovery Reference for CONFIGURE syntax

Configuring Specific Channel Parameters

In addition to configuring parameters that apply to all channels of a particular type, you can also configure parameters that apply to one specific channel. Run the CONFIGURE CHANNEL n command (where n is a positive integer less than 255) to configure a specific channel.

When manually numbering channels, you must specify one or more channel options (for example, MAXPIECESIZE or FORMAT) for each channel. When you use that specific numbered channel in a backup, the configured settings for that channel will be used instead of the configured generic channel settings.

Configure specific channels by number when it is necessary to control the parameters set for each channel separately. This technique could be necessary in the following situations:

  • When running an Oracle Real Application Clusters (Oracle RAC) database in which individual nodes do not have access to the full set of backups. Each channel must be configured with a node-specific connect string so that all backups are accessible by at least one channel.

  • When using a media manager that requires different PARMS settings on each channel.

See Also:

Oracle Real Application Clusters Administration and Deployment Guide to learn about RMAN backups in an Oracle RAC environment

Configuring Specific Channels: Examples

In this example, you want to send disk backups to two different disks. Configure disk channels as follows:

CONFIGURE DEFAULT DEVICE TYPE TO disk;        # backup goes to disk
CONFIGURE DEVICE TYPE disk PARALLELISM 2;     # two channels used in in parallel
CONFIGURE CHANNEL 1 DEVICE TYPE DISK FORMAT '/disk1/%U' # 1st channel to disk1 
CONFIGURE CHANNEL 2 DEVICE TYPE DISK FORMAT '/disk2/%U' # 2nd channel to disk2
BACKUP DATABASE; # backup - first channel goes to disk1 and second to disk2

Assume a different case in which you have two tape drives and want each tape drive to use tapes from a different tape media family. Configure your default output device and default tape channels as shown in the following example to parallelize the database backup.

Example 6-1 Configuring Channel Parallelism for Tape Devices

CONFIGURE DEFAULT DEVICE TYPE TO sbt;    # backup goes to sbt
CONFIGURE DEVICE TYPE sbt PARALLELISM 2; # two sbt channels allocated by default
# Configure channel 1 to pool named first_pool
CONFIGURE CHANNEL 1 DEVICE TYPE sbt 
  PARMS 'ENV=(OB_MEDIA_FAMILY=first_pool)'; 
# configure channel 2 to pool named second_pool
CONFIGURE CHANNEL 2 DEVICE TYPE sbt 
  PARMS 'ENV=(OB_MEDIA_FAMILY=second_pool)'; 
BACKUP DATABASE; # first stream goes to 'first_pool' and second to 'second_pool'

Note that in Example 6-1, the backup data is divided between the two tape devices. Each configured channel backs up roughly half the total data.

Relationship Between CONFIGURE CHANNEL and Parallelism Setting

The PARALLELISM setting is not constrained by the number of specifically configured channels. For example, if you back up to 20 different tape devices, then you can configure 20 different SBT channels, each with a manually assigned number (from 1 to 20) and each with a different set of channel options. In such a situation, you can set PARALLELISM to any value up to the number of devices, in this instance 20.

RMAN always numbers parallel channels starting with 1 and ending with the PARALLELISM setting. For example, if the default device is SBT and parallelism is set to 3, then RMAN names the channels as follows:

ORA_SBT_TAPE_1
ORA_SBT_TAPE_2
ORA_SBT_TAPE_3

RMAN always uses the name ORA_SBT_TAPE_n even if you configure DEVICE TYPE sbt (not the synonymous sbt_tape). RMAN always allocates the number of channels specified in PARALLELISM, using specifically configured channels if you have configured them and generic channels if you have not. Note that if you configure specific channels with numbers higher than the parallelism setting, RMAN will not use these channels.

See Also:

"RMAN Channels" to learn about channels

Configuring Advanced Backup Options

"Configuring the Environment for RMAN Backups" explains the basics for configuring RMAN to make backups. This section explained more advanced configuration options. This section contains the following topics:

Configuring the Maximum Size of Backup Sets

In tape backups, it is possible for a multiplexed backup set to span multiple tapes, which means that blocks from each datafile in the backup set are written to multiple tapes. If one tape of a multivolume backup set fails, then you lose the data on all the tapes rather than just one. If a backup is not a multisection backup, then a backup set always includes a whole datafile rather than a partial datafile. You can use MAXSETSIZE to specify that each backup set should fit on one tape rather than spanning multiple tapes.

The CONFIGURE MAXSETSIZE command limits the size of backup sets created on a channel. This CONFIGURE setting applies to any channel, whether manually allocated or configured, when the BACKUP command is used to create backup sets. The default value is given in bytes and is rounded down to the lowest kilobyte value.

The value set by the CONFIGURE MAXSETSIZE command is a default for the given channel. You can override the configured MAXSETSIZE value by specifying a MAXSETSIZE option for an individual BACKUP command.

Assume that you issue the following commands at the RMAN prompt:

CONFIGURE DEFAULT DEVICE TYPE TO sbt;
CONFIGURE CHANNEL DEVICE TYPE sbt PARMS 'ENV=(OB_MEDIA_FAMILY=first_pool)'; 
CONFIGURE MAXSETSIZE TO 7500K;
BACKUP TABLESPACE users;
BACKUP TABLESPACE tools MAXSETSIZE 5G;

The results will be as follows:

  • The backup of the users tablespace uses the configured SBT channel and the configured default MAXSETSIZE setting of 7500K.

  • The backup of the tools tablespace uses the MAXSETSIZE setting of 5G used in the BACKUP command.

Configuring the Maximum Size of Backup Pieces

Backup piece size is an issue in situations where it exceeds the maximum file size permitted by the file system or media management software. You can use the MAXPIECESIZE parameter of the CONFIGURE CHANNEL or ALLOCATE CHANNEL command to limit the size of backup pieces.

For example, to always limit the backup piece size to 2 GB or less, you can configure the automatic DISK channel as follows and then run BACKUP DATABASE:

CONFIGURE CHANNEL DEVICE TYPE DISK MAXPIECESIZE 2G;
BACKUP DATABASE;

Note:

In version 2.0 of the media management API, media management vendors can specify the maximum size of a backup piece that can be written to their media manager. RMAN respects this limit regardless of the settings you configure for MAXPIECESIZE.

See Also:

Oracle Database Backup and Recovery Reference to learn about the CONFIGURE CHANNEL ... MAXPIECESIZE command

Configuring Backup Duplexing

You can use the CONFIGURE ... BACKUP COPIES command to specify how many copies of each backup piece should be created on the specified device type for the specified type of file. This type of backup is known as a duplexed backup set. The CONFIGURE settings for duplexing only affect backups of datafiles, control files, and archived logs into backup sets, and do not affect image copies.

Note:

A control file autobackup is never duplexed.

RMAN can duplex backups to either disk or tape, but cannot duplex backups to tape and disk simultaneously. When backing up to tape, ensure that the number of copies does not exceed the number of available tape devices. The following examples show possible duplexing configurations:

# Makes 2 disk copies of each datafile and control file backup set
# (autobackups excluded)
CONFIGURE DATAFILE BACKUP COPIES FOR DEVICE TYPE DISK TO 2;
# Makes 3 copies of every archived redo log backup to tape
CONFIGURE ARCHIVELOG BACKUP COPIES FOR DEVICE TYPE sbt TO 3;

To return a BACKUP COPIES configuration to its default value, run the same CONFIGURE command with the CLEAR option, as in the following example:

CONFIGURE DATAFILE BACKUP COPIES FOR DEVICE TYPE sbt CLEAR;

By default, CONFIGURE ... BACKUP COPIES is set to 1 for each device type.

Note:

If you do not want to create a persistent copies configuration, then you can specify copies with the BACKUP COPIES and the SET BACKUP COPIES commands.

See Also:

Configuring Tablespaces for Exclusion from Whole Database Backups

In some cases you may want to exclude specified tablespace part of the regular backup schedule, as in these cases:

  • A tablespace is easy to rebuild, so it is more cost-effective to rebuild it than back it up every day.

  • A tablespace contains temporary or test data that you do not need to back up.

  • A tablespace does not change often and therefore should be backed up on a different schedule from other backups.

You can run CONFIGURE EXCLUDE FOR TABLESPACE to exclude the specified tablespace from the BACKUP DATABASE command. The exclusion condition applies to any datafiles that you add to this tablespace in the future.

For example, you can exclude testing tablespaces cwmlite and example from whole database backups as follows:

CONFIGURE EXCLUDE FOR TABLESPACE cwmlite;
CONFIGURE EXCLUDE FOR TABLESPACE example;

If you run the following command, then RMAN backs up all tablespaces in the database except cwmlite and example:

BACKUP DATABASE;

You can still back up the configured tablespaces by explicitly specifying them in a BACKUP command or by specifying the NOEXCLUDE option on a BACKUP DATABASE command. For example, you can enter one of the following commands:

# backs up the whole database, including cwmlite and example
BACKUP DATABASE NOEXCLUDE;
BACKUP TABLESPACE cwmlite, example;  # backs up only cwmlite and example

You can disable the exclusion feature for cwmlite and example as follows:

CONFIGURE EXCLUDE FOR TABLESPACE cwmlite CLEAR;
CONFIGURE EXCLUDE FOR TABLESPACE example CLEAR;

RMAN includes these tablespaces in future whole database backups.

Configuring the Backup Compression Algorithm

RMAN supports binary compression of backup sets. The supported algorithms are BZIP2 (default) and ZLIB. The BZIP2 algorithm is optimized for maximum compression, whereas the ZLIB algorithm is optimized for CPU efficiency. BZIP2 consumes more CPU resource than ZLIB, but will usually produce more compact backups. The COMPATIBLE initialization parameter must be set to 11.0.0 or higher for ZLIB compression, which requires the Oracle Advanced Compression option.

You can configure the compression algorithm with the following syntax, where alg_name is a placeholder specifying either BZIP2 or ZLIB.

Example 6-2 Configuring the Backup Compression Algorithm

CONFIGURE COMPRESSION ALGORITHM TO 'alg_name';

Configuring Backup Encryption

For improved security, you can configure backup encryption for RMAN backup sets. Encrypted backups cannot be read if they are obtained by unauthorized users. This feature requires the Enterprise Edition of the database.

About Backup Encryption

The V$RMAN_ENCRYPTION_ALGORITHMS view contains a list of encryption algorithms supported by RMAN. If no encryption algorithm is specified, then the default encryption algorithm is 128-bit Advanced Encryption Standard (AES). Note that RMAN encryption requires the COMPATIBLE initialization parameter at a target database to be at least 10.2.0.

RMAN offers the following encryptions modes:

  • Transparent Encryption of Backups

    This is the default mode and uses the Oracle wallet. A wallet is a password-protected container used to store authentication and signing credentials, including private keys, certificates, and trusted certificates needed by SSL.

  • Password Encryption of Backups

    This mode uses only password protection. You must provide a password when creating and restoring encrypted backups.

  • Dual Mode Encryption of Backups

    This mode requires either the wallet or a password.

Note:

Wallet-based encryption is more secure than password-based encryption because no passwords are involved. You should use password-based encryption only when absolutely necessary because your backups need to be transportable.

Encrypted backups are decrypted automatically during restore and recovery, as long as the required decryption keys are available. Each backup set gets a separate key. The key is stored in encrypted form in the backup piece. The backup is decrypted with keys obtained by means of a user-supplied password or the Oracle wallet.

To create encrypted backups on disk with RMAN, the database must use the Advanced Security Option. The Oracle Secure Backup SBT is the only supported interface for making encrypted RMAN backups directly to tape. RMAN issues an ORA-19916 error if you attempt to create encrypted RMAN backups using an SBT library other than Oracle Secure Backup. Note that the Advanced Security Option is not required when making encrypted backups using the Oracle Secure Backup SBT.

When you use the BACKUP BACKUPSET command with encrypted backup sets, the backup sets are backed up in encrypted form. Because BACKUP BACKUPSET copies an already-encrypted backup set to disk or tape, no decryption key is needed during BACKUP BACKUPSET. The data is never decrypted during any part of the operation. The BACKUP BACKUPSET command can neither encrypt nor decrypt backup sets.

See Also:

Oracle Database Advanced Security Administrator's Guide for details about configuring the Oracle wallet
Transparent Encryption of Backups

Transparent encryption can create and restore encrypted backups with no DBA intervention, as long as the required Oracle key management infrastructure is available. Transparent encryption is best suited for day-to-day backup operations, where backups are restored to the same database from which they were created. Transparent encryption is the default for RMAN encryption.

When using transparent encryption, you must first configure an Oracle wallet for each database, as described in Oracle Database Advanced Security Administrator's Guide. Transparent backup encryption supports both the encrypted and autologin forms of the Oracle wallet. When using the Oracle wallet, the wallet must be opened before you can perform backup encryption. When using the autologin wallet, encrypted backup operations can be done at any time, because the autologin wallet is always open.

Caution:

If you use an autologin wallet, then do not back it up along with your encrypted backup data, because users can read the encrypted backups if they obtain both the backups and the autologin wallet. It is safe to back up the Oracle wallet because that form of the wallet cannot be used without the wallet password.

After the Oracle wallet is configured, encrypted backups can be created and restored with no further DBA intervention. Note that if some columns in the database are encrypted with Transparent Data Encryption, and if those columns are backed up using backup encryption, then those columns will be encrypted a second time during the backup. When the backup sets are decrypted during a restore, the encrypted columns are returned to their original encrypted form.

Because the Oracle key management infrastructure archives all previous master keys in the Oracle wallet, changing or resetting the current database master key will not affect your ability to restore encrypted backups performed with an older master key. You can reset the database master key at any time. RMAN will always be able to restore all encrypted backups that were ever created by this database.

Caution:

If you lose your Oracle wallet, then you will be unable to restore any transparently-encrypted backups.
Password Encryption of Backups

Password encryption requires that the DBA provide a password when creating and restoring encrypted backups. Restoring a password-encrypted backup requires the same password used to create the backup.

Password encryption is useful for backups that will be restored at remote locations, but which must remain secure in transit. Password encryption cannot be persistently configured. You do not need to configure an Oracle wallet if password encryption will be used exclusively.

Caution:

If you forget or lose the password that you used to encrypt a password-encrypted backup, then you will be unable to restore the backup.

To use password encryption, use the SET ENCRYPTION ON IDENTIFIED BY password ONLY command in your RMAN scripts.

Dual Mode Encryption of Backups

Dual-mode encrypted backups can be restored either transparently or by specifying a password. Dual-mode encrypted backups are useful when you create backups that are normally restored onsite using the Oracle wallet, but which occasionally need to be restored offsite, where the Oracle wallet is not available.

When restoring a dual-mode encrypted backup, you can use either the Oracle wallet or a password for decryption.

Caution:

If you forget or lose the password that you used to encrypt a dual-mode encrypted backup and you also lose your Oracle wallet, then you will be unable to restore the backup.

To create dual-mode encrypted backup sets, specify the SET ENCRYPTION ON IDENTIFIED BY password command in your RMAN scripts.

Configuring RMAN Backup Encryption Modes

You can use the CONFIGURE command to persistently configure transparent encryption of backups. You can use the command to specify the following:

  • Whether to use transparent encryptions for backups of all database files

  • Whether to use transparent encryptions for backups of specific tablespaces

  • Which algorithm to use for encrypting backups

You can also use the SET ENCRYPTION command to perform the following actions:

  • Override the encryption settings specified by the CONFIGURE ENCRYPTION command. For example, you can use SET ENCRYPTION OFF to create an unencrypted backup, even though a database is configured for encrypted backups.

  • Set a password for backup encryption, persisting until the RMAN client exits. Because of the sensitive nature of passwords, RMAN does not permit configuration of passwords that persist across RMAN sessions.

Note that no persistent configuration controls whether archived redo log backups are encrypted. Backup sets containing archived redo log files are encrypted if any of the following are true:

  • SET ENCRYPTION ON is in effect at the time that the archive log backup is being created.

  • Encryption is configured for backups of the whole database or at least one tablespace.

This behavior ensures that the redo associated with any encrypted backup of a datafile is also encrypted.

To configure the environment so that all RMAN backups are encrypted:

Set up the Oracle wallet as explained in Oracle Database Advanced Security Administrator's Guide.

Issue the following RMAN command:

CONFIGURE ENCRYPTION FOR DATABASE ON;

At this stage, all RMAN backup sets created by this database will use transparent encryption by default.

You can explicitly override the persistent encryption configuration for an RMAN session with the following command:

SET ENCRYPTION ON;

The encryption setting remains in effect until you issue the SET ENCRYPTION OFF command during an RMAN session, or change the persistent setting again with the following command:

CONFIGURE ENCRYPTION FOR DATABASE OFF;

Configuring the Backup Encryption Algorithm

You can use the CONFIGURE command to persistently configure the default algorithm to use for encryption when writing backup sets. Possible values are listed in V$RMAN_ENCRYPTION_ALGORITHMS. The default algorithm is AES 128-bit.

To configure the default backup encryption algorithm:

Start RMAN and connect to a target database and a recovery catalog (if used).

Ensure that the target database is mounted or open.

Execute the CONFIGURE ENCRYPTION ALGORITHM command, specifying a valid value from V$RMAN_ENCRYPTION_ALGORITHMS.ALGORITHM_NAME.

The following example configures the algorithm to AES 256-bit encryption:

CONFIGURE ENCRYPTION ALGORITHM TO 'AES256';

Configuring Auxiliary Instance Datafile Names

Assume that you are performing tablespace point-in-time recovery (TSPITR) or performing data transfer with RMAN. In this case, you may want to set the names of datafiles in the auxiliary instance before starting the TSPITR or database duplication. The command is as follows, where datafileSpec identifies some datafile by its original name or datafile number, and filename is the new path for the specified file:

CONFIGURE AUXNAME FOR datafileSpec TO 'filename';

For example, you might configure a new auxiliary name for datafile 2 as follows:

CONFIGURE AUXNAME FOR DATAFILE 2 TO '/newdisk/datafiles/df2.df';

As with other settings, the CONFIGURE command setting persists across RMAN sessions until cleared with CONFIGURE ... CLEAR, as shown in the following example:

CONFIGURE AUXNAME FOR DATAFILE 2 CLEAR;

If you are performing TSPITR or running the DUPLICATE command, then by using CONFIGURE AUXNAME you can preconfigure the filenames for use on the auxiliary database without manually specifying the auxiliary filenames during the procedure.

When renaming files with the DUPLICATE command, CONFIGURE AUXNAME is an alternative to SET NEWNAME command. The difference is that after you set the AUXNAME the first time, you do not need to reset the filename when you issue another DUPLICATE command; the AUXNAME setting remains in effect until you issue CONFIGURE AUXNAME ... CLEAR. In contrast, you must reissue the SET NEWNAME command every time you rename files.

See Chapter 20, "Performing RMAN Tablespace Point-in-Time Recovery (TSPITR)," for more details on using CONFIGURE AUXNAME in connection with TSPITR, and Chapter 23, "Duplicating a Database," for more on using CONFIGURE AUXNAME in performing database duplication.

Configuring the Snapshot Control File Location

When RMAN needs to resynchronize the recovery catalog with a read-consistent version of the control file, it creates a temporary snapshot control file. RMAN needs a snapshot control file when resynchronizing with the recovery catalog or when making a backup of the current control file.

The default location for the snapshot control file is platform-specific and depends on the Oracle home of each target database. For example, the default filename on some Linux platforms is $ORACLE_HOME/dbs/snapcf_@.f. If a flash recovery area is configured for a target database, then the default location for the snapshot control file is not the flash recovery area.

Viewing the Configured Location of the Snapshot Control File

You can see the current snapshot location by running the SHOW command. This example shows a snapshot location that is determined by the default rule:

RMAN> SHOW SNAPSHOT CONTROLFILE NAME;
CONFIGURE SNAPSHOT CONTROLFILE NAME TO '/oracle/dbs/snapcf_trgt.f'; # default

This example shows a snapshot control file that has a nondefault filename:

RMAN> SHOW SNAPSHOT CONTROLFILE NAME;
CONFIGURE SNAPSHOT CONTROLFILE NAME TO '/oracle/oradata/trgt/snap_trgt.ctl';

Setting the Location of the Snapshot Control File

Use the CONFIGURE SNAPSHOT CONTROLFILE NAME TO 'filename' command to change the name of the snapshot control file. Subsequent snapshot control files that RMAN creates use the specified filename.

For example, start RMAN and then enter:

CONFIGURE SNAPSHOT CONTROLFILE NAME TO '/oracle/oradata/trgt/snap_trgt.ctl';

You can also set the snapshot control file name to a raw device.

To reset the snapshot control file location to the default, run the CONFIGURE SNAPSHOT CONTROLFILE LOCATION CLEAR command.

See Also:

Configuring RMAN for Use with a Shared Server

RMAN cannot connect to a target database through a shared server dispatcher. RMAN requires a dedicated server process. If your target database is configured for shared server, then you must modify your Oracle Net configuration to provide dedicated server processes for RMAN connections.

To ensure that RMAN does not connect to a dispatcher when a target database is configured for a shared server, the net service name used by RMAN must include (SERVER=DEDICATED) in the CONNECT_DATA attribute of the connect string.

Oracle Net configuration varies greatly from system to system. The following procedure illustrates only one method. This scenario assumes that the following service name in the tnsnames.ora connects to a target database using the shared server architecture, where inst1 is a value of the SERVICE_NAMES initialization parameter:

inst1_shs =
  (DESCRIPTION=
    (ADDRESS=(PROTOCOL=tcp)(HOST=inst1_host)(port=1521))
    (CONNECT_DATA=(SERVICE_NAME=inst1)(SERVER=shared))
  ) 

To use RMAN with a shared server:

Create a net service name in the tnsnames.ora file that connects to the nonshared SID. For example, enter:

inst1_ded =
  (DESCRIPTION=
    (ADDRESS=(PROTOCOL=tcp)(HOST=inst1_host)(port=1521))
    (CONNECT_DATA=(SERVICE_NAME=inst1)(SERVER=dedicated))
  )

Start SQL*Plus and then connect using both the shared server and dedicated server service names to confirm the mode of each session.

For example, connect with SYSDBA privileges to inst1_ded and then execute the following SELECT statement (sample output included):

SQL> SELECT SERVER 
  2  FROM   V$SESSION 
  3  WHERE  SID = (SELECT DISTINCT SID FROM V$MYSTAT);

SERVER   
---------
DEDICATED
1 row selected.

To connect to a shared server session, you could connect with SYSDBA privileges to inst1_shs and then execute the following SELECT statement (sample output included):

SQL> SELECT SERVER 
  2  FROM   V$SESSION 
  3  WHERE  SID = (SELECT DISTINCT SID FROM V$MYSTAT);

SERVER   
---------
SHARED 
1 row selected.
  

Start RMAN and connect to the target database using the dedicated service name. Optionally, connect to a recovery catalog. For example, enter:

% rman
RMAN> CONNECT TARGET SYS@inst1_ded 

target database Password: password
connected to target database: INST1 (DBID=39525561)

RMAN> CONNECT CATALOG rman@catdb

See Also:

Your platform-specific Oracle documentation and your Oracle Database Net Services Reference for a complete description of Oracle Net connect string syntax

Enabling Lost Write Detection

A data block lost write occurs when an I/O subsystem acknowledges the completion of the block write, while in fact the write did not occur in the persistent storage. On a subsequent block read, the I/O subsystem returns the stale version of the data block, which might be used to update other blocks of the database, thereby corrupting it.

You can set the DB_LOST_WRITE_PROTECT initialization parameter to TYPICAL or FULL so that a database records buffer cache block reads in the redo log. The default setting is NONE. When the parameter is set to TYPICAL, the instance logs buffer cache reads for read/write tablespaces in the redo log, but not read-only tablespaces. When set to FULL, the instance also records reads for read-only tablespaces. The performance overhead for TYPICAL mode is roughly 5-10%. For Oracle RAC the overhead for FULL mode can increase to 20%.

Lost write detection is most effective when used with Data Guard. In this case, you set DB_LOST_WRITE_PROTECT in both primary and standby databases. When a standby database applies redo during managed recovery, it reads the corresponding blocks and compares the SCNs with the SCNs in the redo log. If the block SCN on the primary database is lower than on the standby database, then it detects a lost write on the primary database and throws an external error (ORA-752). If the SCN is higher, it detects a lost write on the standby database and throws an internal error (ORA-600 [3020]). In either case, the standby database writes the reason for the failure in the alert log and trace file.

To repair a lost write on a primary database, you must initiate failover to the standby database. To repair a lost write on a standby database, you must re-create the entire standby database or restore a backup of only the affected files.

Enabling lost write detection is also useful when not using Data Guard. In this case, you can encounter a lost write in two ways: during normal database operation or during media recovery. In the first case, there is no deterministic way to detect the error. Indirect symptoms such as inconsistent tables cannot be unambiguously traced to the lost write. If you retained a backup made before the suspected lost write, however, then you can restore this backup to an alternative location and recover it. To diagnose the problem, recover the database or tablespace to the SCN of the stale block read, which will generate the lost write error (ORA-752).

In the case of a lost write error encountered during media recovery, the only response is to open the database with the RESETLOGS option. The database is in consistent state, but all data after the RESETLOGS SCN is lost. Note that if you recover a backup made after database creation, you have no guarantee that other stale blocks have not already corrupted the database. This possibility exists because the restored backup may have been made after an earlier lost write. To guarantee that no lost writes have corrupted the database, you must perform media recovery from database creation, which is not a practical strategy for most database environments.

See Also: